main.cpp:13: warning: (if this is not what you intended, make sure the function
template has already been declared and add <> after the function name here) //what does this mean???
-Wno-non-template-friend disables this warning.

I refered some books but none of them have examples on outside class defenitions of templated friend functions
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I did a board search and I found this:http://cboard.cprogramming.com/showt...riend+function
and also this in the same post: